Hybrid aircraft propulsion power plants

ABSTRACT

An aircraft power plant has: a hybrid propulsion system having an electric motor, an output shaft drivingly connectable to a thrust generator, a combustion engine, a compressor, and a transmission having a first transmission drive path and a second transmission drive path selectively engageable to the first transmission drive path, the electric motor and the compressor in driving engagement with the first transmission drive path, the combustion engine and the output shaft in driving engagement with the second transmission drive path.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority from U.S. patent application 62/826,189filed on Mar. 29, 2019, the entire contents of which are incorporated byreference herein.

TECHNICAL FIELD

The application relates generally to hybrid aircraft propulsion powerplants and in particular to means for operating a propulsion engine onthe aircraft in an auxiliary power unit mode.

BACKGROUND OF THE ART

An auxiliary power unit (APU) is typically used in aircraft to providevarious functionalities, such as an independent power drive as anadjunct to a primary engine. For example an APU may be used to provideback-up power to the electrical power system of an aircraft, while theaircraft is grounded or during an in-flight emergency. APU are typicallyshut down when power form the primary engine is available.

SUMMARY

In a first aspect, there is provided an aircraft power plant comprising:a hybrid propulsion system having an electric motor, an output shaftdrivingly connectable to a thrust generator, a combustion engine, acompressor, and a transmission having a first transmission drive pathand a second transmission drive path selectively engageable to the firsttransmission drive path, the electric motor and the compressor indriving engagement with the first transmission drive path, thecombustion engine and the output shaft in driving engagement with thesecond transmission drive path.

In a second aspect, there is provided an aircraft power plantcomprising: a hybrid propulsion system having an electric motor, anoutput shaft drivingly connectable to a thrust generator, a combustionengine, a compressor, and a transmission defining a drive path, thecompressor drivingly engaged to the electric motor via the drive path,the output shaft and the combustion engine drivingly engaged to thedrive path in a propulsion mode of the transmission and disengaged fromthe drive path in an Auxiliary Power Unit (APU) mode.

In a third aspect, there is provided a method of operating an aircraftpower plant having a hybrid propulsion system including an electricmotor, an output shaft drivingly connectable to a thrust generator, acombustion engine, and a compressor, the method comprising: in anAuxiliary Power Unit (APU) mode, transmitting a first rotational inputfrom the electric motor to the compressor along a drive path whilemaintaining a second rotational input from the combustion engineseparate from the output shaft; and in a propelling mode, transmittingthe second rotational input to the output shaft via the drive path todrive the output shaft with both of the combustion engine and theelectric motor.

DESCRIPTION OF THE DRAWINGS

Reference is now made to the accompanying figures in which:

FIG. 1 is a schematic view of a hybrid engine arrangement suitable foruse as a main propulsion power plant on a hybrid electric aircraft;

FIG. 2 is a schematic view of an aircraft power plant operable in an APUmode and a propulsion mode in accordance with one embodiment;

FIG. 3 is a schematic view of another aircraft power plant operable inan APU mode and a propulsion mode in accordance with one embodiment;

FIGS. 4a and 4b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 4a ) and in an APU mode (FIG. 4b )in accordance with one embodiment;

FIGS. 5a and 5b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 5a ) and in an APU mode (FIG. 5b )in accordance with one embodiment;

FIGS. 6a and 6b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 6a ) and in an APU mode (FIG. 6b )in accordance with one embodiment;

FIGS. 7a and 7b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 7a ) and in an APU mode (FIG. 7b )in accordance with one embodiment;

FIGS. 8a and 8b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 8a ) and in an APU mode (FIG. 8b )in accordance with one embodiment;

FIGS. 9a and 9b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 9a ) and in an APU mode (FIG. 9b )in accordance with one embodiment;

FIGS. 10a and 10b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 10a ) and in an APU mode (FIG. 10b )in accordance with one embodiment;

FIGS. 11a to 11c are schematic views of another aircraft power plantoperable in a first APU mode (FIG. 11a ), in a second APU mode (FIG. 11b), and in a propulsion mode (FIG. 11c ) in accordance with oneembodiment;

FIGS. 12a to 12c are schematic views of another aircraft power plantoperable in a first APU mode (FIG. 12a ), in a second APU mode (FIG. 12b), and in a propulsion mode (FIG. 12c ) in accordance with oneembodiment;

FIG. 13 is a schematic view of another aircraft power plant operable inan APU and a propulsion mode in accordance with one embodiment;

FIGS. 14a and 14b are schematic views of another aircraft power plantoperable in a propulsion mode (FIG. 14a ) and in an APU mode (FIG. 14b )in accordance with one embodiment;

FIGS. 15a to 15g are schematic view of another aircraft power plant(FIG. 15a ) operable in a prolusion mode and in an APU mode andillustrating relative rotations (FIGS. 15b to 15g ) of differentcomponents of a planetary gear train in function of the operation mode.

DETAILED DESCRIPTION

FIG. 1 illustrates an example of a propulsion engine for an aircraft.More particularly, FIG. 1 illustrates a power plant 10 suitable toprovide thrust to a prime mover propulsor of the aircraft. It isunderstood that the power plant 10 can adopt various configurations.According to the illustrated example, the power plant 10 is configuredas a turboprop engine for driving a propeller 11. However, the powerplant 10 could also be configured as a turboshaft engine for driving ahelicopter rotor or any other thrust producing device or thrustgenerator or rotor for providing motive power to the aircraft.

The power plant 10 may generally have a combustion engine 12, aturbocharger 14, a boost compressor 16, an electric motor/generator(electric motor) 18, which may be powered by an on-board battery pack(not shown), and a compounding or power combining gearbox 20 having amain output drivingly connected to a reduction gearbox (RGB) 22 that is,in turn, drivingly connected to the propeller 11. As shown in FIG. 1,the compounding gearbox 20 can also act as an accessory gearbox byhaving secondary outputs drivingly connected to accessories 24 (e.g.fuel pump, oil pump, starter, etc.). Also, the (RGB) 22 could beintegrated to the compounding gearbox 20.

The combustion engine 12 may be part of a module that may include one ormore internal combustion engines engaged to a common shaft (not shown)to provide a first drive input to the compounding gearbox 20. In aparticular embodiment, the internal combustion engine(s) is/are rotaryinternal combustion engine(s), for example Wankel engine(s); asdescribed in U.S. Pat. No. 9,353,680 granted to Villeneuve et al., inLents et al.'s U.S. Pat. No. 7,753,036 issued Jul. 13, 2010 or asdescribed in Julien et al.'s U.S. Pat. No. 7,775,044 issued Aug. 17,2010, or as described in Thomassin et al.'s U.S. patent publication No.2015/0275749 published Oct. 1, 2015, or as described in Bolduc et al.'sU.S. patent publication No. 2015/0275756 published Oct. 1, 2015, theentire contents of all of which are incorporated by reference herein. Itis however understood that other types of internal combustion enginesmay alternately be used.

In accordance with another possible embodiment, the combustion enginemodule 12 may comprise a gas turbine engine, for instance, a singlespool turbine engine or other turbine engine configurations where a loadremains on the turbine when disconnected from the propeller.

Various possible schematics for the power plant are described hereinbelow. Like reference numerals in different drawings refer to likeelements of the power plants. It is understood that a plurality ofvariations or combinations of the disclosed power plants arecontemplated. For instance, more than one electric motor 18 may be used,either to compound their rotational input or for driving differentelements. In some cases, the one or more electric motor may be used asgenerators to recharge the batteries.

State differently, an electric motor module may include one or moreelectric motors 18. The electric motor module may include one or moremotors or rotor/stator combinations capable of producing a rotatingoutput torque when provided with suitable electric power. The electricmotor module 18 provides a second drive input to the compounding gearbox20. In this way, the joint power of the electric motor module 18 and thecombustion engine module 12 may be used to drive a common load (e.g. thepropeller 11, the compressor 16, the accessories 24, etc.).

The turbocharger 14 has a turbine section 14 a which may provide a thirddrive input to the compounding gearbox 20. Accordingly, the turbocharger14 may compound power with the combustion engine 12 and the electricmotor 18 to drive a common load. Alternatively, the turbocharger 14could only be used to drive the compressor 16. It is noted that thecompressor 16 may be mounted to the shaft of the turbine section of theturbocharger 14 or not. The turbocharger 14 may further includes acompressor 14 b for compressing air from an environment and having anoutlet fluidly connected to an air inlet of the combustion engine 12.The compressor and the turbine 14 b, 14 a of the turbocharger 14 may bein driving engagement on a shaft 14 c, which is shown schematically witha dashed line in FIG. 1.

It is noted that the power plant 10 could further comprise a dedicatedcabin bleed compressor in addition to the boost compressor 16. Thededicated cabin compressor could be sized for the specific needs of thecabin air only.

In operation, the compressor 16 feeds compressed air to the inlet(s) ofthe combustion engine module 12. The combustion engine module 12receives the pressurized air from the compressor 16 and burns fuel athigh pressure to provide energy. Mechanical power produced by thecombustion engine module 12 is inputted into the compounding gearbox 20.The combustion engine module 12 provides an exhaust flow in the form ofexhaust pulses of high pressure hot gas exiting at high peak velocity.The outlet of the combustion engine module 12 is in fluid communicationwith an inlet of the turbine section of the turbocharger 14, andaccordingly the exhaust flow from the combustion engine module 12 issupplied to the turbine section of the turbocharger 14 before beingexhausted to atmosphere.

As will be seen hereinafter, various mechanical arrangements are hereincontemplated for allowing an aircraft power plant such as the variousexemplary power plants described herein above to also be used as anauxiliary power unit (APU) to generate electrical power and/orpressurized air to the aircraft to support the aircraft pneumatic systemwhile the aircraft is on the ground. With the following defined methodsof incorporating an APU mode, the boost compressor 16 or a dedicatedcabin bleed compressor may be driven to provide bleed air services tothe aircraft while the propeller 11 is disengaged (i.e. the propeller isnot rotated, the propeller is in AU mode).

Depending upon the proposed configurations, the APU mode activates thecompressor using either the electric motor module 18 alone, thecombustion engine module 12 alone, the combustion engine module 12 alonewith the electric motor module 18 in generator mode to assist inrecharging the batteries or using both the electric motor module 18 andthe combustion engine module 12. Incorporating an APU function in thedesign of an hybrid engine allows to eliminate the need for a separateaircraft APU or a separate electrically driven compressor, which savesequipment cost, reduces the aircraft weight and reduces the aircraftequipment maintenance time and cost.

In all of the embodiments described below, the aircraft power plantsinclude a hybrid propulsion system S including at least one combustionengine 12 and at least one electric motor 18. A transmission, differentembodiments of which are described herein below, is used for receivingthe rotational inputs of the different components, such as thecombustion engine and the electric motor, operatively connected theretoand to transmits required inputs to other components in need of rotationinputs, such as the load compressor and the thrust generator 11 (e.g.,propeller, fan, rotor, and so on).

In the following lines, when two different components are said to be“drivingly engaged” to one another it implies that a rotational input istransmitted from one of the two components to the other. In some cases,two different components are “disengaged” from one another. This may beachieved having a brake in a drive, load, or transmission paththerebetween.

In the following lines, the expression “transmission” may be any deviceable to receive a rotational input via an input and to transmit it to anoutput. The transmission may include, for instance, gears meshed withone another, pulleys and belts, clutches, one-way clutches, brakes, andso on.

The expressions “combustion engine”, or “heat engine”, may be any engineable to generate a torque based on a combustion of a hydrocarbon, suchas a fossil fuel. The fuel may be a synthetic fuel derived from othersources. Such a fuel may be, for instance, alcohol or bio diesel. Thecombustion engine may be, for instance, a reciprocating engine such as apiston engine, a rotary engine, a gas turbine engine of any types suchas a turbofan, a turboshaft, and a turbojet. The combustion engine maydefine one or more combustion chambers of varying volume.

The expression “hybrid” means that the power plant includes at least oneelectric motor and at least one combustion engine so that the powerprovided by the power plant comes from two different sources (e.g.,electrical and chemical/thermal).

The expression “compressor” may include any machine able to increase apressure of a fluid. For instance, the compressor may be a centrifugalcompressor, an axial compressor, a combination of the an axialcompressor and a centrifugal compressor. The compressor may include oneor more compression stages.

The expression “output shaft” may include an output of the transmissionthat is drivingly engageable to the thrust generator. The output shaftmay be directly engaged to the thrust generator or may be engaged tosaid generator via the RGB 22.

Interruptible Path

Referring now to FIG. 2, an aircraft power plant in accordance with oneembodiment is shown generally at 100 a. The power plant 100 a includes atransmission 20. In the embodiment shown, the transmission 20 defines afirst drive path 20 a and a second drive path 20 b, which are shown indashed lines. Herein, the first and second drive paths 20 a, 20 b aregear trains selectively engageable to one another via a one-way clutch20 c or an electromechanical clutch 20 d. A one-way clutch is alsoreferred to as an over-running clutch. An active clutch providingpositive engagement and disengagement of the drive may be used. Such anactive clutch may be an electromechanical clutch, a hydraulic clutch, ora mechanically actuated clutch. The first and second drive paths 20 a,20 b can consist of a number of spur gears or the like. Idler gears maybe provided in each drive path to accommodate shaft spacing when neededand/or to change rotational direction when required. In this particularembodiment, the electric motor module 18 comprises a first electricmotor 18 a and a second electric motor 18 b. Two electric motors 18 a,18 b may be useful when the compressor 16 is physically located too farfrom the main electric motor 18 b to be easily geared to it.

The first electric motor 18 a, the combustion engine module 12, and theturbine section of the turbocharger 14 are drivingly engaged to thefirst drive path 20 a. As schematically illustrated in FIG. 2, the firstdrive path 20 a has an output drivingly connected to the RGB 22 of thepropeller 11. The second electric motor 18 b is drivingly engaged tosecond drive path 20 b. The second drive path 20 b has a first outputfor the accessories 24 and a second output for the compressor 16. Themechanical or drive path from the compressor 16 to the turbocharger 14may be opened by the clutch 20 c or 20 d to allow the hybrid propulsionsystem S to be used in an APU mode. Indeed, the a rotational inputprovided to the compressor gear may be isolated from the turbochargerturbine 14 because of the one-way clutch 20 c (or when theelectromechanical clutch 20 d is disengaged) but the turbocharger 14 candrive the combustion engine 12.

In the embodiment shown, the electric motor 18 b and the compressor 16are in driving engagement with the second transmission drive path 20 b.The combustion engine 12 and an output shaft 23 are in drivingengagement with the first transmission drive path 20 a. Herein, theoutput shaft 23 corresponds to a shaft that is drivingly engaged to theRGB 22 and acts as an input to the RGB 22.

The one-way clutch 20 c or the electromechanical clutch 20 d areengagement devices operable in an APU mode and in a propulsion mode. Inthe APU mode, the two drive paths 20 a, 20 b are disengaged from oneanother. In the propulsion mode, the two drive paths 20 a, 20 b are indriving engagement with each other so that both of the combustion engine12 and the electric motor 18 b are drivingly engaged to the compressor16 and to the output shaft 23.

The electromechanical clutch 20 d may have an engaged configuration thatcorresponds to the propulsion mode and a disengaged configurationcorresponding to the APU mode. For the one-way clutch 20 c, a rotationalinput of the combustion engine 12 is transmittable to the compressor 16a but a rotation input from the electric motor 18 b is isolated form theoutput shaft 23 in the APU mode.

Stated differently, the transmission 20 defines a drive path 20 b viawhich the compressor 16 is drivingly engaged to the electric motor 18 b.The output shaft 23 and the combustion engine 12 are drivingly engagedto the drive path 20 b in a propulsion mode of the transmission 20 anddisengaged from the drive path 20 b in an Auxiliary Power Unit (APU)mode.

In a propulsion mode, power inputted in the first drive path 20 a by thefirst electric motor 18 a, the combustion engine module 12 and theturbocharger 14 is transferred to the second drive path 20 b via theclutch 20 b or 20 d, thereby allowing the compressor 16 and theaccessories 24 to be driven. The second electric motor 18 b may bedriven so as to be used as a generator or, alternatively, it may bepowered to provide an additional power input. For instance, if an on-offtooth clutch is used, the second electric motor 18 b could be sized togenerate mechanical shaft power in excess of the compressor needs andthe excess power could be used to contribute power to the combustionengine module output shaft via the engaged electromechanical clutch 20 dand the turbocharger turbine gear. It is noted that the second electricmotor 18 b could be mounted on a same shaft as the compressor 16.

In the APU mode, the second electric motor 18 b is used to drive thecompressor 16 without driving components downstream of the clutch 20 cor 20 d and, thus, may feed the aircraft air systems while the otherpower sources (e.g. electric motor 18 a, combustion engine module 12 andturbocharger turbine 14) are inactive. The lubrication, scavenging andcooling of the second electric motor 18 b and compressor 16 aremaintained by the accessory drive connected by the gear train commonwith the second electric motor 18 b output. Optionally, a brake 20 e maybe engaged on the propeller shaft. The brake is optional as theresistance on the rotation of the inactive combustion engine module 12may be enough in APU mode to maintain the propeller at rest.

FIG. 3 illustrates another possible arrangement of the power plant 100 aof FIG. 2 and is shown generally at 100 b. In the present embodiment,the transmission 20 comprises first and second drive paths 20 a, 20 b(e.g. first and second gear trains) connected by a clutch, such aselectromechanical clutch 20 d. In this embodiment, however, the electricmotor module 18 comprises a single electric motor 18 a disposed in thevicinity of the compressor 16 and having an input gear forming part ofthe second drive path 20 b. The electric motor 18 a is sized for thetotal electric power requirement (which is in excess of the compressorneeds).

The combustion engine module 12 and the turbocharger turbine 14 a mayprovide drive inputs to the first drive path 20 a. The first drive path20 a has an output drivingly connected to the RGB 22 via the outputshaft 23 for driving the propeller 11. The second drive path 20 b has ondrive input from the electric motor 18 a and two outputs, a first one tothe compressor 16 and a second one to the accessories 24.

To start the combustion engine module 12, the clutch 20 d is firstengaged and then the electric motor 18 a is used as a starter to crankthe combustion engine 12. In normal propulsion operating mode, theelectric motor 18 a drives the compressor 16 via the electric motor gearand the compressor gear and delivers excess power to the combustionengine 12 via the engaged clutch 20 d. The electric motor 18 a thuscompounds power with the turbine 14 a of the turbocharger 14 and thecombustion engine 12 to drive the propeller 11 via the output shaft 23.

In the APU mode, the clutch 20 d is disengaged and, thus, the firstdrive path 20 a is disengaged from the second drive path 20 b, therebyallowing the compressor 16 to be driven by the electric motor 18 awithout driving the propeller 11 (i.e. the first drive path and, thus,the propeller 11 is disconnected from the electric motor 18 a). As such,compressor air delivery may be provided to the aircraft systems (e.g.,environmental control systems) without driving the propeller 11.

An optional brake 20 e could be provided and activated in the APU modeto restrain the propeller 11 against rotation. Also a one-wayover-running or sprag clutch could be used instead of the on-off clutch20 d. This may however require the use of an additional dedicated startsystem for the internal combustion engine module 12 instead of beingable to use the electric motor 18 a.

Referring now to FIGS. 4a and 4b another embodiment of an aircraft powerplant is shown generally at 100 c and is depicted in the propulsion modein FIG. 4a and in the APU mode in FIG. 4b . The power plant 100 cincludes a transmission 120 defining a drive path 120 a. The electricmotor 18 and the compressor 16 are in driving engagement with the drivepath 120 a. The combustion engine 12 and the output shaft 23 aredrivingly engageable to one another along a second drive path 120 b viaoperation of two clutches 120 c, 120 d.

The first and second drive paths 120 a, 120 b are engageable to oneanother via two clutches 120 c and 120 d. The first clutch 120 c isconnected between the combustion engine 12 and the first drive path 120a. The second clutch 120 d is connected between the output shaft 23 andthe first drive path 120 a. In the present embodiment, the second drivepath 120 b is created by the engagement of one or both of the twoclutches 120 c, 120 d. In the propulsion mode, both of the two clutches120 c, 120 d are in their engaged configuration such that the combustionengine 12 and the electric motor 18 drivingly engages both of thecompressor 16 and the propeller 11. A brake 120 e may be used to preventrotation of the propeller 11 in the APU configuration. This may bedesirable if the clutch system has a small amount of residual drag whendisengaged, or to prevent wind-milling of the propeller due to externalair currents when the drive is disconnected.

In this embodiment, the combustion engine 12 may be integrated with aninternally geared turbocharger 14 to provide a common input to thetransmission 120. Only a single electric motor may be used in thisembodiment and may have an output shaft engaged to an electric motorgear g1 in meshing engagement with a main output gear g2 of the gearbox20. The output gear g2 is drivingly connected to the RGB 22 via thefirst clutch 120 d. The output gear g2 is also in meshing engagementwith the compressor gear g3 of the compressor 16 and with the accessorygear g4 of the accessories 24.

As shown in FIG. 4a , in the propulsion mode, both the first and secondclutches 120 c, 120 d are engaged and the brake 120 e on the propellerdrive is disengaged. As such, the electric motor 18 and the combustionengine and turbocharger 12, 14 compound powers to drive the propeller11, the compressor 16 and the accessories 24 via the gear train of thetransmission 120.

As shown in FIG. 4b , in the APU mode, the second clutch 120 d isdisengaged, thereby disconnecting the propeller 11 from the transmission120. The first clutch 120 c on the combustion engine 12, 14 may beengaged for operation of the compressor 16 with both the combustionengine module 12, 14 and the electric motor 18 or disengaged foroperation with the electric motor 18 only. Alternatively, the APU modemay be with the combustion engine module 12, 14 only. This may beachieved by engaging the first clutch 120 c and by not energising theelectric motor 18. The brake 120 e on the propeller drive may be engagedin all APU mode scenarios. The electric motor 18 may be used as agenerator in both of the APU and propulsion mode. The generator 18 maytherefore be used to recharge batteries of the aircraft equipped withthe power plant 100 c or for providing electrical power to systems ofsaid aircraft.

For operating the aircraft power plant 10, in an Auxiliary Power Unit(APU) mode, a first rotational input from the electric motor 18 istransmitted to the compressor 16 along a drive path while a secondrotational input from the combustion engine 12 is maintained separatefrom the output shaft 23; and in a propelling mode, the secondrotational input is transmitted to the output shaft 23 via the drivepath to drive the output shaft 23 with both of the combustion engine 12and the electric motor 18.

In the embodiment shown, transmitting the second rotational input to theoutput shaft 23 includes drivingly engaging the combustion engine 12 tothe drive path via the clutch.

Parallel Paths

Referring now to FIGS. 5a and 5b , another embodiment of an aircraftpower plant is shown generally at 100 d. The power plant 100 d has atransmission 220 defining first and second drive paths 220 a, 220 b. Thecombustion engine 12 and the output shaft 23 are in driving engagementwith the first drive path 220 a. The electric motor 18 and thecompressor 16 drivingly engageable to one another via either one of thefirst drive path 220 a or the second drive path 220 b. The selection ofwhich of the first and second drive paths 220 a, 220 b is used may befunction of a direction of rotation of the electric motor 18.

In the embodiment shown, the first drive path 220 a comprises anelectric motor gear 41, a combustion engine gear 43 and a compressorgear 45. The electric motor 18 and the combustion engine 12 respectivelyprovides first and second drive inputs to the first drive path 220 a viatheir respective gears. The first drive path 220 a has a first outputdrivingly connected to the RGB 22 of the propeller 11 via the combustionengine 12 and output shaft 23 and a second output to the compressor 16by virtue of the driving engagement between the combustion engine gear43 and the compressor gear 45.

The second drive path 220 b comprises a number of idler gears 51, 53,55, 57. It is noted that the number of idler gears is selected so thatthe compressor 16 may be always driven in the same direction of rotationirrespective of whether the power is transferred via the first or viathe second drive paths 220 a, 220 b. The first or input idler gear 51 isconnected to the electric motor 18 shaft via a first one-way clutch 60.The first one-way clutch 60 may allow torque to be transferred from theelectric motor 18 to the input idler gear 51 of the second drive path220 b in one rotation direction only (the clockwise (CW) direction inthe illustrated example). However, if the output shaft of the electricmotor 18 is driven in a counter clockwise (CCW) direction as shown inFIG. 5a , no torque is transferred from the electric motor 18 to theinput gear 51 of the second drive path 220 b and, thus, the second drivepath 220 b is disabled.

A second one way-clutch 62 may be provided on the electric motor shaftto allow torque to be transferred from the electric motor gear 41 to thecombustion engine gear 43 in only one rotation direction of the electricmotor gear (the CCW direction in the illustrated example).

A third one-way clutch 64 is provided at the output end of the seconddrive path 220 b to prevent a torque to be transferred from thecompressor shaft to the output idler gear 57 of the second drive path220 b when the combustion engine 12 is operated in the propulsion mode.

A fourth one-way clutch 66 is provided between the compressor 16 and thecombustion engine 12. The fourth one-way clutch 66 may allow power to betransferred from the combustion engine gear 43 to the compressor gear 45but may prevent the opposite. That is, when the compressor 16 is driventhrough the second drive path 220 b; the torque transferred to thecompressor 16 is not transferred to the combustion engine gear 43.

As shown in FIG. 5a , in the propulsion mode, the electric motor 18 isdriven in the CCW direction. Under such circumstances, no torque istransferred to the second drive path 220 b due to the presence of thefirst one-way clutch 60. The second drive path 220 b is thus disabled.The power is transmitted through the first drive path 220 a as allowedby the second gear clutch 62 due the rotation direction of the electricmotor. The power of the electric motor 18 and the combustion engine 12may be combined to drive the propeller 11 and the compressor 16 via thefirst drive path 220 a. The third one-way clutch 64 prevents torquetransfer from the compressor shaft to the last idler gear 57 of thesecond drive path 220 b.

As shown in FIG. 5b , in the APU mode, the electric motor 18 is drivenin a CW direction, thereby disabling the first drive path 220 a via thesecond one-way clutch 62. When the electric motor 18 is driven in the CWdirection the first one-way clutch 60 allows a torque to be transferredfrom the electric motor gear 41 to the second drive path 220 b.Therefore, by rotating the electric motor 18 in the CW direction, thepower is transmitted through the second drive path 220 b rather than thefirst drive path 220 a. In this way, the electric motor 18 may beenergized to drive the compressor without driving the propeller 11.

Moreover, in the APU mode, the electric motor 18 drives the compressor16 and bypasses the combustion engine 12. In other words, the combustionengine 12 is excluded from the second drive path 220 b. This allows theelectric driving of the compressor 16 without having to operate thecombustion engine 12 (combustion engine may remain shut down) andwithout having to rotate a shaft of the combustion 12 when said engineis shut down. In other words, the electric motor 18 may not have toovercome a load resulting from the combustion engine 12 beinginoperative.

Note that the initial choice of direction of rotation is arbitrary butwas given in the example to illustrate the equipment with a uniquedirection of rotation. Also, it is understood that the combustionengine, turbine, compressor and the pumps are provided with the samerotation direction in both the propulsion mode and the APU mode.

Referring now to FIGS. 6a and 6b , another embodiment of an aircraftpower plant is shown generally at 100 e. In the embodiment shown, thetransmission 320 comprises an epicyclic gear train 30 comprising a sungear 30 a, planetary gears 30 b meshed with the sun gear 30 a androtatably mounted on a carrier 30 c and a ring gear 30 d meshed with theplanet gears 30 b. In the embodiment shown, a first brake 32 isselectively engageable with the ring gear 30 d to restrain the ring gear30 against rotation. A second brake 34 is selectively engageable withthe carrier 30 c to restrain the carrier 30 c against rotation.

The transmission 320 defines first and second drive paths 320 a, 320 bbeing parallel to one another. As will be seen herein after, the brakepositions on the planetary 30 dictates which of the parallel drive paths320 a, 320 b is used.

The first drive path 320 a is used to transmit rotation of the electricmotor 18 to the compressor 16 while bypassing the combustion engine 12and the output shaft 23 in the APU mode (FIG. 6b ). The second drivepath 320 b is used to transmit rotation of the electric motor 18 and ofthe combustion engine 12 to both of the output shaft 23 and to thecompressor 16 in the propulsion mode (FIG. 6a ).

In the embodiment shown, the first drive path 320 a includes a firstgear 21 a in meshing engagement with the ring gear 30 d, a secondintermediate gear 21 b and a third gear 21 c in meshing engagement withthe second gear 21 b. The second drive path 320 b comprises a combustionengine gear 21 d on the shaft of the combustion 12, the combustionengine gear 21 d in meshing engagement with the carrier 30 c and with acompressor gear 21 e mounted to the compressor shaft of the compressor16. A first one-way clutch 320 c is provided on the compressor shaftbetween the third gear 21 c of the first drive path 320 a and thecompressor gear 21 e of the second drive path 320 b. A second one-wayclutch 320 d is provided between the combustion engine module gear 21 dand the compressor gear 21 e.

In the embodiment shown, the electric motor 18 is in driving engagementwith the sun gear 30 a. As schematically shown in FIG. 6a , thecombustion engine 12 may be integrated with an internally gearedturbocharger 14 as discussed hereinabove.

As shown in FIG. 6a , in the propulsion mode, the first brake 32 isengaged with the ring gear 30 d to lock the same against rotation andthe electric motor 18 is energized to drive the sun gear 30 a, which inturn drives the carrier 30 c and thus the compressor 16 via thecombustion engine gear 21 d and the compressor gear 21 e of the seconddrive path 320 b. The gears 21 a, 21 b and 21 c of the first drive path320 a, which are engaged with the ring gear 30 d, remain stationary sothe torque is only transferred via the second drive path 320 b. Thecombustion engine 12 is drivingly engaged to the carrier 30 c andprovides propulsion power to the propeller 11 via the output shaft 23.The electric motor 18 can compound power with the combustion engine 12to drive the propeller 11. More particularly, the electric motor 18 maybe used to start the combustion engine module 12 and to add power duringtake-off. The second one way clutch 320 d on the compressor shaft allowspower to be transferred from the combustion engine gear 21 d to thecompressor gear 21 e to drive the compressor 16. The first one wayclutch 320 c however may prevent the compressor gear 21 e from drivingthe third idler gear 21 c of the first drive path 320 a.

As shown in FIG. 6b , in the APU mode, the second brake 34 is engaged tolock the carrier 30 c. In this way, the second drive path 320 b isdrivingly disconnected from the electric motor 18. The electric motor 18on the sun gear 30 a drives the ring gear 30 d, which in turn drives thegears 21 a, 21 b and 21 c of the first drive path 320 a. The firstone-way clutch 320 c allows power to be transferred from the third idlergear 21 c to the compressor shaft to drive the compressor 16. However,the second one-way clutch 320 d may prevent the compressor gear 21 efrom driving the combustion engine gear 27 21 d. The combustion enginegear 21 d is engaged with the carrier 30 c, which is locked, so itsrotational speed may be zero. It is noted that the carrier brake 34 alsoacts like a propeller brake and could be located anywhere on themechanical path between the propeller 11 and the carrier 30 c (i.e. itdoes not have to directly engage with the carrier). In view of theforegoing, it may be appreciated that in the APU mode, the electricmotor 18 may be energized to drive the compressor 16 while the propeller11 is disabled.

The transmission 320 allows the use of solely one of the two drive paths320 a, 320 b at a time. In the propulsion mode, only the second drivepath 320 b is used and the first drive path 320 a remains at rest. Inthe APU mode, only the first drive path 320 a is used and the seconddrive path 320 b remains at rest. It is understood that any othersuitable variations of the transmission 320 are contemplated withoutdeparting from the scope of the present disclosure. For instance, theelectric motor 18 may be drivingly engaged to the carrier 30 c, thefirst drive path 320 a may stem from the sun gear 30 a and the seconddrive path 320 b may stem from the ring gear. Any other variations arecontemplated. Note that, the combustion engine 12 is excluded from thefirst drive path 320 a.

For operating the aircraft power plant 10, in an Auxiliary Power Unit(APU) mode, a first rotational input is transmitted from the electricmotor 18 to the compressor 16 along a first drive path while maintaininga second rotational input from the combustion engine 12 separate fromthe output shaft 23; and in a propelling mode, the first rotationalinput and the second rotational input are compounded on a second drivepath different than the first drive path to drive the output shaft withboth of the combustion engine 12 and the electric motor 18.

In the embodiment shown, transmitting the first rotational input fromthe electric motor 18 to the compressor 16 includes rotating theelectric motor 18 in a first rotation direction and compounding thefirst rotational input and the second rotational input includes rotatingthe electric motor 18 in a second rotation direction opposite the firstrotation direction.

Differential

Referring now to FIGS. 7a and 7b , another embodiment of an aircraftpower plant is shown generally at 100 f. The power plant 100 f includesa transmission 420 having an epicyclic gear train 30 comprising a sungear 30 a, planetary gears 30 b, a carrier 30 c and a ring gear 30 d. Afirst brake 32 and a second brake 34 are provided to dictate whatequipment may be driven. The first brake 32 may be engaged to lock thesun gear 30 a. The second brake 34 may be engaged to lock the carrier 30c.

The electric motor 18 a is drivingly engaged with the ring gear 30 d.The ring gear 30 d is also drivingly engaged with the compressor 16 viathe compressor gear. The turbine 14 and the combustion engine module 12are drivingly engaged to the carrier 30 c. The output shaft 23 is indriving engagement with the carrier 30 c.

The transmission 420 is operable in an Auxiliary Power Unit (APU) mode(FIG. 7b ) in which the electric motor 18 is in driving engagement withthe compressor 16 via the planetary gear train 30 while the combustionengine 12 is disengaged from the output shaft 23, and a propulsion mode(FIG. 7a ) in which the combustion engine 12 and the electric motor 18is in driving engagement with the output shaft 23 via the planetary geartrain 30.

The planetary gear train 30 may define a first input, which hereincorresponds to the ring gear 30 d, a second input, which hereincorresponds to the planet carrier 30 c, a first output, which hereincorresponds to the ring gear 30 d, and a second output, which hereincorresponds to the carrier 30 c. In the embodiment shown, the firstinput is drivingly engageable with the electric motor 18, the secondinput is drivingly engageable with the combustion engine 12, the firstoutput is drivingly engageable with the compressor 16, and the secondoutput is drivingly engageable with the output shaft 23. The first inputis drivingly engaged to the first output while being disengaged from thesecond output in the APU mode. The second input is in driving engagementwith the second output in the propulsion mode.

As shown in FIG. 7a , in the propulsion mode, the first brake 32 isactivated to lock the sun gear 30 a. The second brake 34 is disengagedand, thus, the carrier 30 c is free to rotate. The combustion engine 12,the turbine 14 and the electric motor 18 may all contribute to power todrive the compressor 16 and the propeller 11. The output shaft of thegearbox 20 may be centered and connected to the carrier 30 c to providea drive input to the RGB 22 driving the propeller 11. The planetarysystem provides a first stage of speed reduction for the propeller 11.The compressor 16 is driven by the electric motor 18, the combustionengine 12 and the turbine 14 via the ring gear 30 d.

In the embodiment shown, and in the propulsion mode, a first drive path420 a is created by the transmission 420 and extends from the combustionengine 12 to the propeller 11 and the compressor 16 via the planetcarrier 30 c and the ring gear 30 d.

As shown in FIG. 7b , in the APU mode, the first brake 32 is disengagedto allow the sun gear 30 a to rotate. The second brake 34 is engaged tolock the carrier 30 c. The combustion engine 12 and the turbine 14cannot rotate since they are engaged with the carrier 30 c. The electricmotor 18 is energized to drive the compressor 16 through the ring gear30 d. The sun gear 30 a spins freely in direction opposite to the ringgear 30 d. A generator (not shown) could be connected to the sun gear totake advantage of the rotation of the sun gear to recharge battery ofthe electric motor 18. Alternatively, a dedicated cabin bleed compressorcould be connected to the sun gear 30 a. Since the gearbox output to thepropeller 11 is on the carrier 30 c, the second brake 34 may also act asa propeller brake to prevent rotation of the propeller 11 when theengine is used in the APU mode. According to this embodiment, only theelectric motor 18 may be used in the APU mode to drive the compressor16.

In the embodiment shown, and in the APU mode, a second drive path 420 bis created by the transmission 420 and extends from the electric motor18 to the compressor 16 via the ring gear 30 d.

In the embodiment shown, the combustion engine 12 is drivinglyengageable to the output shaft 23 via the first drive path 420 a. Theelectric motor 18 is drivingly engageable to the compressor 16 via thesecond drive path 420 b. The planetary gear train 30 has an AuxiliaryPower Unit (APU) configuration in which the output shaft 23 isdisengaged from both of the first and second drive paths 420 a, 420 band in which a rotational input of the electric motor 18 is transmittedto the compressor 16 via the second drive path 420 b and in a propellingconfiguration in which the output shaft 23 is drivingly engaged to thecombustion engine 12 via the first drive path 420 a and in which thefirst drive path 420 a is drivingly engaged to the second drive path 420b.

It is understood that many variations are contemplated. For instance,the combustion engine 12 and the output shaft 23 may be drivinglyconnected to the sun gear; the electric motor 18 and the compressor 16may be drivingly connected to the ring gear. Any other variations arecontemplated without departing from the scope of the present disclosure.Selection of the most effective combination in any situation may dependon the requirements of the system components in terms of speeds, torquesand mechanical packaging to minimize weight and/or volume.

It will be appreciated that the second brake 34 may be omitted if atorque required to induce rotation of the thrust generator is greaterthan that to induce rotation of the compressor 16. For instance, andreferring to FIG. 7b , in the APU mode, the sun gear 30 a is free torotate. Allowing the carrier to rotate, that is by either omitting thesecond brake 34 or by having the second brake 34 in the disengagedconfiguration, may allow the electric motor to drive the compressor 16since the torque may travel along a path of least resistance. In thiscase, the path of least resistance may extend from the electric motor 18to the load compressor 16 and may not reach the thrust generator becauseof relative inertias and internal frictional loads compared to that ofthe compressor 16. Although many of the power plants described hereinmay use a plurality of brakes, it will be appreciated that, in somecases as explained above, some of those brakes may be omitted.

Referring now to FIGS. 8a and 8b , another embodiment of an aircraftpower plant is shown generally at 100 g. This embodiment is a furthervariant which essentially differs from the embodiment of FIGS. 7a and 7b, in that the combustion engine 12 and the turbine 14 are drivinglyengaged with the ring gear 30 d instead of the carrier 30 c. Accordingto this variant, the combustion engine 12 and the turbine 14 alone or acombination of the combustion module 12, the turbine 14 and the electricmotor 18 may be used in the APU mode to drive the compressor 16. Whenused alone in the APU mode, the combustion engine 12 may be used todrive the compressor 16 and the electric motor 18 in a generator mode tohelp recharge the batteries while at the same time providing pressurisedair for the cabin. The pressurizing of the air for the cabin and/or therecharging of the batteries may be carried without rotation of thepropeller or thrust generator that may hinder freedom of movement aroundthe aircraft during ground servicing.

Referring now to FIGS. 9a and 9b , another embodiment of an aircraftpower plant is shown generally at 100 h. This embodiment is a furthervariant which essentially differs from the embodiment shown in FIGS. 8aand 8b in that two one-way clutches 71, 73 have been added to allow theelectric motor 18 to be used alone in the APU mode to drive thecompressor 16. However, a starter (not shown) may be needed to start thecombustion engine 12 since the one-way clutches 71, 73 may prevent theelectric motor 18 from driving the combustion engine 12 and the turbine14 via the ring gear 30 d.

The first one way clutch 71 is provided on the combustion engine outputshaft whereas the second one-way clutch 73 is provided on the turbine 14output shaft. In the propulsion mode, the first and second one-wayclutches 71, 73 allow the combustion engine 12 and the turbine 14 torotate the ring gear 30 d. However, in the APU mode, that is when thecarrier 30 c is locked, the one-way clutches 71, 73 prevent a torque tobe transferred from the ring gear 30 d to the combustion engine 12 andthe turbine 14. This may allow the electric motor 18 to be used alone todrive the compressor 16. However, if desired, like the previousembodiment, the combustion engine 12, the turbine 14 and the electricmotor 18 may be used together to drive the compressor 16 to providepressurized air to the aircraft cabin. Alternatively, the combustionengine module could be used alone to drive the compressor 16 in the APUmode.

Referring now to FIGS. 10a and 10b , another embodiment of an aircraftpower plant is shown generally at 100 i. In this case, the combustionengine 12 and the turbine 14 are combined to offer a common input to theplanetary gear train 30. An on-off clutch 74 (e.g. electromechanicalclutch, hydraulic, or toothed clutch) is provided to selectively connector disconnect the combined combustion engine and turbine module 12, 14from the ring gear 30 d. The on-off clutch 74 replaces the one-wayclutches 71 and 73 in the embodiment of FIGS. 9a and 9b . By replacingthe one-way clutches with an on-off clutch, the electric motor 18 a canstill be used as a starter for the combined combustion engine andturbine module 12, 14.

In the propulsion mode, the clutch 74 is engaged. In this way, theelectric motor 18 and the combined combustion engine and turbine 12, 14may be used to drive the compressor 16 and the accessories 24 via thering gear 30 d. The torque from the ring gear 30 d is transmitted to thecarrier 30 c via the planet gears 30 b to drive the propeller 11.

In the APU mode, the clutch 74 may be engaged or disengaged depending ifit is desired to use the electric motor 18 alone to drive the compressor16. By disengaging the clutch 74, the electric motor 18 may be usedalone to drive the compressor 16. When the clutch 74 and the brake 34are engaged, the combined combustion engine and turbine 12, 14 may beused alone or with the electric motor 18 to drive the compressor 16. Ifthe combined combustion engine and turbine module 12, 14 is used alone,the electric motor 18 may be used as a generator to recharge thebatteries.

It is understood that combustion engine 12 and the turbo charger turbine14 do not have to be integrated. They could have separate drive inputsinto the planetary gear train 30 as for instance shown in FIGS. 8a and8b . Separate on-off clutches would then be used on the combustionengine module 12 and on the turbine 14.

Referring now to FIGS. 11a to 11c , another embodiment of an aircraftpower plant is shown generally at 100 j. In the embodiment shown, afirst planetary gear train 30 and a second planetary gear train 130 areused and connected to one another by respective sun gears 30 a, 130 a. Ashaft may be used to connect the two sun gears 30 a, 130 a together. Theelectric motor 18 and the compressor 16 are drivingly connected to thesun gear 30 a of the first planetary gear train 30. The combustionengine module 12 and the turbine 14 are drivingly connected to the ringgear 130 d of the second planetary gear train 130. The output shaft 23is drivingly connected to the carrier 30 c of the first planetary geartrain 30. A first brake 32 is engageable with the ring gear 30 d of thefirst planetary gear train 30. A second brake 34 is engageable with thecarrier 30 c of the first planetary gear train 30. A third brake 36 isengageable with the ring gear 130 d of the second planetary gear train130. A fourth brake 38 is engageable with the carrier 130 c of thesecond planetary gear train 130. The brake positions dictate whatequipment may be driven.

In the APU mode, the second and third brakes 34, 36 are engaged torespectively lock the carrier 30 c of the first planetary gear train 30(and thus the propeller 11) and the ring gear 130 d of the secondplanetary gear train 130. The electric motor 18 is then energized. Theelectric motor gear drives the sun gear 30 a of the first planetary geartrain 30, which, in turn, drives the compressor gear and, thus, thecompressor 16. The rotation of the sun gear 30 a of the first planetarygear train 30 is transmitted to the ring gear 30 d via the planet gears30 b. The ring gear 30 d of the first planetary 30 rotates freely as itis not linked to any load. The rotation of the sun gear 30 a of thefirst planetary 30 is transmitted to the sun gear 130 a of the secondplanetary 130 via the shaft connecting the two sun gears 30 a, 130 a.The rotation of the sun gear 130 a of the second planetary 130 istransmitted to the carrier 130 c via the planet gears 130 b. Like thering gear of the first planetary 30, the carrier 130 c of the secondplanetary 130 spins freely. The ring gear 130 d of the second planetary130 is locked by the third brake 36, thereby preventing rotation of thecombustion engine gear and of the turbine gear. Accordingly, thecompressor 16 is driven by the electric motor 18 only in the APU mode.

However, as shown in FIG. 11b , the combustion engine 12 could be usedwith the electric motor 18 to drive the compressor 16. This may be doneby disengaging the third brake 36 from the ring gear 130 d of thesecondary planetary 130 and by engaging the fourth brake 38 with thecarrier 130 c of the second planetary 130. In such a configuration, thecombustion engine module 12 and the turbine 14 can drive the sun gear130 a through the ring gear 130 d. The sun gear 130 a on the secondplanetary 130 drives the sun gear 30 a on the first planetary 30,thereby driving the compressor 16. The combustion engine module 12 canalso be used to recharge the batteries in the APU mode by driving theelectric motor 18 in a generator mode.

FIG. 11c illustrates the propulsion mode. The second brake 34 isdisengaged to unlock the carrier 30 c of the first planetary 30, therebyunlocking the propeller 11. The first brake 32 is engaged to lock thering gear 30 d of the first planetary 30. On the second planetary 130,the fourth brake 38 is engaged to lock the carrier 130 c. The thirdbrake 36 is disengaged to unlock the ring gear 130 d. The electric motor18 a may be energized to crank and start the combustion engine 12. Theelectric motor 18 drives the sun gear 30 a of the first planetary 30and, thus, the sun gear 130 a of the second planetary 130 via the sungear interconnecting shaft. The sun gear 130 a of the second planetary130 drives the ring gear 130 d which rotates the combustion engine gearand the turbine gear. Once the starting procedure is completed, thecombustion engine module 12 and the turbine 14 can drive the sun gear130 a through the ring gear 130 d. The sun gear 130 a on the secondplanetary 130 drives the sun gear 30 a on the first planetary 30,thereby providing power to the compressor 16. The torque transferredfrom the combustion engine module 12 and the turbine 14 to the sun gear30 a of the first planetary 30 is transferred to the propeller 11 viathe carrier 30 c of the first planetary 30. Accordingly, the combustionengine module 12, the turbine 14 and the electric motor 18 a may be usedall together to drive the propeller 11 and the compressor 16.Alternatively, the combustion engine module 12 could be used without theelectric motor 18 a to drive both the propeller 11 and the compressor16.

Referring now to FIGS. 12a to 12c , another embodiment of an aircraftpower plant is shown generally at 100 k. FIGS. 12a to 12c show anotherexample of a dual planetary gear train but this time the two planetarygear trains 30, 130 share a “common” ring gear instead of a “common” sungear. The electric motor 18 and the compressor 16 are connected to thering gear 30 d of the first planetary 30. The first brake 32 isengageable with the sun gear 30 a. The second brake 34 is engageablewith the carrier 30 c on which is connected the propeller 11. The thirdbrake 36 is engageable with the sun gear 130 a of the second planetary130. The fourth brake 38 is engageable with the carrier 130 c of thesecond planetary 130. The combustion engine module 12 and the turbine 14are on the carrier 130 c of the second planetary 130.

FIG. 12a illustrates an electric motor only APU mode. In this mode, thesecond brake 34 is engaged to lock the carrier 30 c of the firstplanetary 30 and, thus, the propeller 11. The electric motor 18 candrive the compressor 16 through the ring gear 30 d using battery orground electric source. The sun gear 30 a spins freely in a directionopposite to the ring gear 30 d. On the second planetary 130, the fourthbrake 38 is engaged to lock the carrier 130 c, thereby preventing torquetransmission to the combustion engine gear and the turbine gear. Thering gear 130 d rotates with the ring gear 30 of the first planetary 30as the same are connected to rotate in unison. This causes the sun gear130 a on the second planetary 130 to spin freely. The electric motor 18can thus be used alone to drive the compressor 16.

FIG. 12b illustrates a combustion engine and electric motor APU mode inwhich the electric motor 18, the combustion engine 12 and turbine 14 maybe jointly used via the interconnected ring gears 30 d, 130 d to drivethe compressor 16. In this mode, the third brake 36 is engaged to lockthe sun gear 130 a of the second planetary 130 and the fourth brake 38is disengaged to unlock the carrier 130 c.

FIG. 12c illustrates the propulsion mode. The second brake 34 isdisengaged to unlock the carrier 30 c and, thus, the propeller 11. Thefirst and third brakes 32 and 36 are engaged to lock the sun gears 30 a,130 a of the first and second planetary gear trains 30, 130,respectively. The combustion engine 12 and the turbine 14 may be used todrive the propeller 11 and the compressor 16. Alternatively, thecombustion engine 12, the turbine 14 and the electric motor 18 may beused. As may be appreciated from FIG. 12c , power from the combustionengine 12 and the turbine 14 may be transferred from the secondplanetary 130 to the first planetary 30 via the interconnected ringgears 30 d, 130 d. The rotation of the ring gear 30 d on the firstplanetary 30 is transmitted to the carrier 30 c via the planet gears 30b. The carrier 30 c in turn drives the propeller 11 through the RGB 22.The electric motor 18 may be energized to compound power with thecombustion engine 12 and the turbine 14 via the ring gear 30 d.

Referring now to FIG. 13, another embodiment of an aircraft power plantis shown generally at 100 l. FIG. 13 illustrates a further variant ofthe above double planetary gear train arrangement. According to thisembodiment, the first and second planetary gear trains 30, 130 aredisposed side-by-side instead of one in front of the other. Thisarrangement may allow better spacing of driving and driven equipment.According to the illustrated variant, the ring gear 30 d, 130 d of thefirst and second planetary gear trains 30, 130 are synchronised throughone or more idler gears (two of which are shown in FIG. 13). Like theprevious embodiments, the propeller 11 is on the carrier 30 c of thefirst planetary 30 and a brake 34 is provided to selectively lock thecarrier 30 c to allow the hybrid propulsion system to be used in an APUmode. The planetary systems may be placed fore and aft of each other byhaving the idler wheels replaced by idler lay-shafts that may providesimilar torque transmission from first to second planetary stages.

Referring now to FIGS. 14a and 14b , another embodiment of an aircraftpower plant is shown generally at 100 m. In the embodiment shown, abrake 34 may be selectively engaged with the carrier 30 c to dictatewhat equipment may be driven. The propeller 11 is once again on thecarrier 30 c. Accordingly, the brake 34 may be engaged to lock thepropeller 11 in the APU mode (FIG. 14b ). According to this variant, theelectric motor unit 18 comprises two electric motors 18 a, 18 b. Thefirst electric motor 18 a is drivingly connected to the ring gear 30 d.The second electric motor 18 b is drivingly engaged with the sun gear 30a. The compressor 16 and the accessories 24 are drivingly connected tothe ring gear 30 d just like in the previous embodiments. Theturbocharger turbine 14 is integrated with the combustion engine 12 toprovide a common input on the carrier 30 c.

In the propulsion mode (FIG. 14a ), the brake 34 is disengaged, therebyallowing the carrier 30 c and the propeller 11 to be driven in rotation.Both electric motors 18 a, 18 b rotate in the same direction and at thesame speed as the integrated combustion engine and turbine 12, 14. Theelectric motor sizing may account for different torque requirements onthe ring gear 30 d and on the sun gear 30 a depending of which of thering gear 30 d or sun gear 30 a drives the accessories 24 and thecompressor 16. In the illustrated embodiment, the first electric motor18 a on the ring gear 30 d is used as a generator such that the carrier30 c sees the same resistance torque from the sun gear 30 a (whichdrives the compressor 16) as from the ring gear 30 d. The electric powerobtained from the first electric motor 18 a may be used to rechargebatteries or to drive the electric motor 18 b on the sun gear 30 a. Bothelectric motors 18 a, 18 b may be used in tandem to vary the compressorspeed on demand.

In the APU mode (FIG. 14b ), both electric motors 18 a, 18 b turn inopposite directions. As an alternative, the compressor 16 andaccessories 24 could be both driven from the same gear (the sun gear orthe ring gear). It is also noted, that this system could work without abrake as the engine control could ensure that the counter rotation ofthe ring gear 30 d and sun gear 30 a results in zero rpm on the carrier30 c. With this system, only the electric motors 18 a, 18 b may be usedin the APU mode.

Referring now to FIG. 15a , another embodiment of an aircraft powerplant is shown generally at 100 n. In the embodiment shown, thepropeller 11 is drivingly connected to the carrier 30 c. A first brake32 may be provided for engagement with the carrier 30 c. A second brake34 may be provided for engagement with the ring gear 30 d. Contrary tothe previous differential embodiments, the compressor 16 and theaccessories 24 are driven by the sun gear 30 a (i.e. not the ring gear30 d). The carrier 30 c is driven by a combustion engine 12, a turbine14, an electrical motor 18 or a combination thereof. The torque on thering gear 30 d may be controlled by a generator 18 b. It is a unitdifferent than the main propulsion electrical motor/generator 18 and maytypically be much smaller. When two electric motors 18, 18 b are used,one may be used for propulsion while the other is used for driving thecompressor 16 and the accessories 24 in the APU mode, the electric motor18 drivingly connected to the carrier 30 c being the one used forpropulsion. The second electric motor 18 b is drivingly connected to thering gear 30 d to drive the compressor 16 on the sun gear 30 a when thecarrier 30 c is locked by the carrier brake 34 (APU mode). Additionalgears may be required between the power sources and the ring gear 30 dto drive the high speed equipment as the ring gear 30 d and the carrier30 c may be limited in speed. It is noted that the brakes or lockingdevices on the carrier 30 c and ring gear 30 d are optional but may beof interest as may be appreciated from the speed distribution diagramsshown in FIGS. 15b to 15g . The accessories 24, including the oil pumps,may be on the same drive as the compressor 16 to ensure properlubrication in APU mode. In the APU mode, only one electric motor 18 bmay be used to drive the compressor 16 and the accessories 24.

FIGS. 15b to 15g respectively illustrate the operation of the embodimentshown in FIG. 15a in the propulsion and the APU mode. The differentdirections of rotation are depicted with arrows. At engine start (FIG.15b ), if there is no load on the generator 18 b, the movement will gowhere there is no resistance. In this case, the movement will go to thering gear and there will be almost no rotation of the sun gear 30 a. Ifsome load is applied to the generator 18 b (FIG. 15c ), the movementswill balance as a function of the respective torques at the ring geargenerator 18 b and at the sun gear compressor 16. Adding generatortorque load at the ring gear 30 d may reduce the ring gear speed andwill increase the compressor speed. As further generator load is applied(FIG. 15d ), the sun gear compressor speed is further increased. Thegenerator 18 b may be able to generate enough torque to compensate thecompressor drive torque. The power produced by the generator 18 b maynot be lost as it may be used to recharge batteries. However, there maybe a 20% loss in the generator power (10% when producing it and send itto the batteries and another 10% when using it back from the batteriesto the electric motor 18. As an optional feature to avoid long periodsof energized generator and reduce the losses, it may be possible to havea brake or solenoid actuated locking pin to maintain the ring gear fixed(FIG. 15e ) at a pre-defined compressor speed. The system may be limitedto the functions defined above but it could also be defined for thegenerator to also have a motor mode (FIG. 15f ). Passed the point of 0RPM on the ring gear, the electrical motor/generator 18 b may rotate thering gear 30 d in the direction opposite the sun gear 30 a which may addmore speed to the sun gear compressor 16. The generator 18 b may be usedas an electric motor when the combustion engine 12 is shut down (FIG.15g ) and rotate the compressor in an APU mode to ventilate an aircraftcabin when the aircraft is on ground electrical power to charge thebatteries. For this APU mode, a brake or lock pin could ensure that thecarrier drive remains at rest. Alternatively, a counter acting torqueapplied at the main motor 18 could be used to maintain the carrier atrest.

General Notes about the Proposed Concepts

Number of gears and direction of rotations may be variable and maydepend on the exact size and speed of driving and driven equipment. Forinstance, in a given design, idler gears may have to be added to obtaina specific distance between shafts or to respect the rotationaldirection of an existing item.

In some of the presented concepts, specific directions of rotation havebeen shown. This was only to demonstrate that the concepts in questionmaintain the desired rotational directions when the system is switchedfrom Propulsion Mode to APU Mode (or vice versa)

When running the power plants in APU mode, lubrication and coolingfunctions are maintained on the portions of the power plant that remainactive. Some of the concepts show where the accessory drive could belocated but for diagram simplicity, it was not shown on all concepts.Where not shown, it may be assumed that the accessory drive will be onthe same section of the gear train as the electric motor or Compressoras these items are always active in APU mode.

To provide air services to the aircraft, the boost compressor 16 wouldneed to be sized to support cabin bleed in addition to engine air flowand the air delivery line would need to have a bleed valve sized andcontrolled accordingly. The bleed valve and bleed controls are notdiscussed.

In some of the concepts with a planetary gear train, the planetarysystem can also constitute a 1st stage of speed reduction, thereforereducing the last ratio required on the RGB.

Embodiments showing two interconnected planetary systems do not show allthe possible interconnection combinations (i.e. sun-sun, sun-carrier orsun to ring and vice versa). However all possibilities are contemplatedas the selection of a specific interconnection would be dependent uponspecific equipment speeds and packaging conditions.

All embodiments using brake or clutch engagements are based onengagement or disengagement when all power sources are turned off exceptfor the electric motor which may be rotated at very slow speed to ensurethe brake or clutch engagement are not falling on a metal to metalcontact point and therefore may be engaged at all times. The presence ofthe electric motor may eliminate the concerns relative to the ability toengage the brakes or clutches from any rest position.

Proper and complete engagement of brakes and clutches may be confirmedby proximity probes or limit switches. For sake of simplification, theuse and positioning of such sensors are not shown in the conceptdiagrams.

In some cases, it may be possible to replace two brakes with a singlebrake that would have two positions, locking one shaft or another shaftdepending on its selected position. As such a brake would have to bedesigned for each specific application; all the concepts are shown withindividual brakes for each location where a braking action would berequired.

Because brakes and clutches are only engaged or disengaged at very lowspeed (could be less than 30 RPM as an indicative value) the brakes andclutches may be of positive engagement types rather than friction types,ensuring a more reliable brake action.

Brakes and clutches may be actuated by hydraulic forces orelectromagnetic forces or any other suitable actuation means.

As mentioned herein before, the compressor may be a dedicated cabinbleed compressor (i.e. not necessarily the boost compressor feedingpressurized air to the internal combustion inlet). This alternative isnot specifically shown in all the drawings but it may be imagined easilysimply by replacing the engine boost compressor in any of the diagramsby a dedicated cabin air compressor and moving the boost compressor to agear driven by the combustion engine module 12.

Another alternative applicable to some of the embodiments would be toadd a dedicated cabin air compressor or an additional generator to thebranch of a planetary system that is left to spin freely when the systemis operated in APU mode. This would be applicable to the embodiments ofFIGS. 7 to 15 where either a sun gear, a carrier or a ring gear spinsfreely when the system is in APU mode. However, this dedicated cabincompressor or generator would be operable in APU Mode only and would notbe available in Propulsion mode.

Most of the embodiments with one electric motor can also accommodatemore than one electric motor if required for redundancy or to address aproblem of limited space that would prevent the installation of onelarger electric motor at the required location in the gear train.

In two electric motor arrangements, typically, one of the electric motorcould be sized to drive the cabin air compressor only in APU mode andthe sum of power of the two electric motors is the same as the electricpower in the schemes with only one electric motor. Both electric motorsassist the ICE/Turbine Modules during Take-off.

The power plants described herein above may allow driving the compressor16 electrically while the combustion engine 12 and other components mayremain at stop. The disclosed power plants are hybrid in that they useboth of an electric motor and a combustion engine whose rotationalinputs may be compounded to drive a thrust generator, such as thepropeller 11. The power plants may be used in propulsion modes and inAPU mode. They may allow the aircraft to avoid using a dedicated APUsince the power plants are able to cater to the function of the APU.

It will be further appreciated that, for any of the embodimentsdisclosed herein, a clutch may be provided between the propeller 11 andthe transmissions to selectively disengage the propeller 11 from thetransmission. This may allow driving the compressor with the electricmotor alone, with the combustion engine alone, or with a combination ofthe electric motor and the combustion engine. In other words, in the APUmodes, one or both of the electric motor and the combustion engine maybe used. The output shaft may be considered as the shaft being indriving engagement with the RGB. Such a clutch may be located betweenthe RGB and the transmission to selectively disconnect the output shaftfrom the transmission.

All the concepts showing a planetary gear train where all the elements(sun gear, ring gear and carrier) can be in rotational movementdepending of applied torques or applied brakes may be consideredconcepts using the differential characteristics of the planetary system.There are other types of differential gear trains that may be usedwithout departing from the scope of the present disclosure.

Embodiments disclosed herein include:

A. An aircraft power plant comprising: a hybrid propulsion system havingan electric motor, an output shaft drivingly connectable to a thrustgenerator, a combustion engine, a compressor, and a transmission havinga first transmission drive path and a second transmission drive pathselectively engageable to the first transmission drive path, theelectric motor and the compressor in driving engagement with the firsttransmission drive path, the combustion engine and the output shaft indriving engagement with the second transmission drive path.

B. An aircraft power plant comprising: a hybrid propulsion system havingan electric motor, an output shaft drivingly connectable to a thrustgenerator, a combustion engine, a compressor, and a transmissiondefining a drive path, the compressor drivingly engaged to the electricmotor via the drive path, the output shaft and the combustion enginedrivingly engaged to the drive path in a propulsion mode of thetransmission and disengaged from the drive path in an Auxiliary PowerUnit (APU) mode.

Embodiments A and B may include any of the following elements, in anycombinations:

Element 1-1: the transmission includes an engagement device operable inan Auxiliary Power Unit (APU) mode in which the first transmission drivepath is disengaged from the second transmission drive path and in whichthe load compressor is driven by the electric motor and in a propulsionmode in which the first transmission drive path is drivingly engaged tothe second transmission drive path and in which both of the combustionengine and the electric motor are drivingly engaged to the compressorand to the output shaft. Element 1-2: the engagement device is anelectromechanical clutch having an engaged configuration correspondingto the propulsion mode and in which the combustion engine is drivinglyengaged to the compressor and having a disengaged configurationcorresponding to the APU mode and in which the combustion engine isdisengaged from the compressor. Element 1-3: the engagement device is aone-way clutch via which a rotational input of the combustion engine istransmittable to the compressor and via which a rotational input fromthe electric motor is isolated from the output shaft. Element 1-4: aturbocharger having a turbocharger compressor and a turbochargerturbine, the turbocharger compressor having an outlet fluidly connectedto an air inlet of the combustion engine, the turbocharger turbinehaving an inlet fluidly connected to an exhaust of the combustionengine. Element 1-5: the turbocharger is mounted on a shaft in drivingengagement with the second transmission drive path. Element 1-6: asecond electric motor in driving engagement with the second transmissiondrive path. Element 1-7: a brake is operatively connected to the outputshaft, the brake in engagement with the output shaft when the firsttransmission drive path is disengaged form the second transmission drivepath. Element 1-8: the second transmission drive path is engageable tothe first transmission drive path via a first clutch and a secondclutch, the first clutch selectively engaging the combustion engine tothe first transmission drive path, the second clutch selectivelyengaging the output shaft to first transmission drive path. Element 1-9:the electric motor is a motor/generator, the combustion engine drivingthe motor/generator in a generator mode when the first transmissiondrive path is drivingly engaged to the second transmission drive path.Element 1-10: the transmission includes an engagement device having adisengaged configuration in which the drive path is disengaged from thecombustion engine and the output shaft while the compressor is driven bythe electric motor and an engaged configuration in which the drive pathis drivingly engaged to the combustion engine and the output shaft.Element 1-11: the engagement device is one of an electromechanicalclutch and a sprag clutch. Element 1-12: a turbocharger having aturbocharger compressor and a turbocharger turbine, the turbochargercompressor having an outlet fluidly connected to an air inlet of thecombustion engine, the turbocharger turbine having an inlet fluidlyconnected to an exhaust of the combustion engine. Element 1-13: theturbocharger is mounted on a shaft drivingly engaged to the outputshaft. Element 1-14: a second electric motor in driving engagement withthe output shaft. Element 1-15: the combustion engine and the outputshaft are engageable to the drive path via a first clutch and a secondclutch. Element 1-16: the electric motor is a motor/generator, thecombustion engine driving the motor/generator in a generator mode whenthe drive path is drivingly engaged to the combustion engine.

C. An aircraft power plant comprising a hybrid propulsion system havingan electric motor, a combustion engine, an output shaft drivinglyconnectable to a thrust generator, a compressor, and a transmissionhaving a first transmission drive path and a second transmission drivepath, the combustion engine and the output shaft in driving engagementwith the first transmission drive path, the electric motor selectivelydrivingly engageable to the compressor via either the first drive pathor via the second drive path.

D. An aircraft power plant comprising a hybrid propulsion system havingan electric motor, a combustion engine, an output shaft drivinglyconnectable to a thrust generator, a compressor, and a transmissionhaving first and second transmission drive paths both extending from theelectric motor to the combustion engine, the output shaft and thecombustion engine drivingly engaged to the first transmission drive pathand excluded from the second transmission drive path, the electric motorselectively drivingly engageable to the compressor via either of thefirst and second transmission drive paths.

Embodiments C and D may include any of the following elements, in anycombinations:

Element 2-1: the transmission includes an engagement device operable inan Auxiliary Power Unit (APU) mode in which a rotational input of theelectric motor is transmitted from the electric motor to the compressorvia the second drive path and in a propulsion mode in which therotational input of the electric motor is transmitted from the electricmotor to the compressor via the first transmission drive path. Element2-2: the engagement device includes four one-way clutches operativelyconnected to the second transmission drive path. Element 2-3: rotationof the electric motor in a first rotational direction drivingly engagesthe electric motor to the compressor via the first transmission drivepath, rotation of the electric motor in a second rotational directionopposite the first rotational direction drivingly engages the electricmotor to the compressor via the second transmission drive path. Element2-4: the transmission includes a planetary gear train having a sun gear,planet gears meshed with the sun gear and rotatably supported by aplanet carrier, and a ring gear meshed with the planet gears, theelectric motor in driving engagement with a first one of the sun gear,the planet carrier, and the ring gear, the first transmission drive pathextending from a second one of the sun gear, the planet carrier, and thering gear, the second transmission drive path extending from a third oneof the sun gear, the planet carrier, and the ring gear, a brakeoperatively connected to the third one of the sun gear, the planetcarrier, and the ring gear and operable in an engaged configuration inwhich the electric motor engages the compressor via the firsttransmission drive path and in a disengaged configuration in which theelectric motor engages the compressor via the second transmission drivepath. Element 2-5: the electric motor is in driving engagement with thesun gear, the first transmission drive path extending from the planetcarrier, the second transmission drive path extending from the ringgear, the brake operatively connected to the ring gear to selectivelylimit or allow rotation of the ring gear. Element 2-6: one-way clutchesare operatively connected to the second transmission drive path. Element2-7: in the engaged configuration of the brake, a rotational input fromthe electric motor is transmitted to the compressor and a rotation ofthe compressor is isolated from the second transmission drive path viaone of the one-way clutches. Element 2-8: the output shaft is in drivingengagement with the planet carrier. Element 2-9: the transmissionincludes an engagement device operable in an Auxiliary Power Unit (APU)mode in which a rotational input of the electric motor is transmitted tothe compressor via the second drive path and bypasses the combustionengine and in a propulsion mode in which the rotational input of theelectric motor is transmitted to the compressor via the firsttransmission drive path. Element 2-10: the engagement device includesfour one-way clutches operatively connected to the second transmissiondrive path. Element 2-11: rotation of the electric motor in a firstrotational direction drivingly engages the electric motor to thecompressor via the first transmission drive path, rotation of theelectric motor in a second rotational direction opposite the firstrotational direction drivingly engages the electric motor to thecompressor via the second transmission drive path. Element 2-12: thetransmission includes a planetary gear train having a sun gear, planetgears meshed with the sun gear and rotatably supported by a planetcarrier, and a ring gear meshed with the planet gears, the electricmotor in driving engagement with a first one of the sun gear, the planetcarrier, and the ring gear, the first transmission drive path extendingfrom a second one of the sun gear, the planet carrier, and the ringgear, the second transmission drive path extending from a third one ofthe sun gear, the planet carrier, and the ring gear, a brake operativelyconnected to the third one of the sun gear, the planet carrier, and thering gear and operable in an engaged configuration in which the electricmotor engages the compressor via the first transmission drive path andin a disengaged configuration in which the electric motor engages thecompressor via the second transmission drive path. Element 2-13: theelectric motor is in driving engagement with the sun gear, the firsttransmission drive path extending from the planet carrier, the secondtransmission drive path extending from the ring gear, the brakeoperatively connected to the ring gear to selectively limit or allowrotation of the ring gear. Element 2-14: two one-way clutches areoperatively connected to the second transmission drive path. Element2-15: in the engaged configuration of the brake, a rotational input fromthe electric motor is transmitted to the compressor and a rotation ofthe compressor is isolated from the second transmission drive path viaone of the two one-way clutches. Element 2-16: the output shaft is indriving engagement with the planet carrier.

E. An aircraft power plant comprising a hybrid propulsion system havingan electric motor, an output shaft drivingly connectable to a thrustgenerator, a combustion engine, a compressor, and a planetary gear trainhaving an Auxiliary Power Unit (APU) mode in which the electric motor isin driving engagement with the compressor via the planetary gear trainwhile the combustion engine is disengaged from the output shaft, and apropulsion mode in which the combustion engine and the electric motorare in driving engagement with the output shaft via the planetary geartrain.

F. An aircraft power plant comprising a hybrid propulsion system havingan electric motor, an output shaft drivingly connectable to a thrustgenerator, a combustion engine, a compressor, and a planetary gear trainoperatively connected to the electric motor, the output shaft, thecombustion engine, and the compressor, the planetary gear train definingfirst and second drive paths, the combustion engine drivingly engageableto the output shaft via the first drive path, the electric motordrivingly engageable to the compressor via the second drive path, theplanetary gear train having an Auxiliary Power Unit (APU) configurationin which the output shaft is disengaged from both of the first andsecond drive paths and in which a rotational input of the electric motoris transmitted to the compressor via the second drive path and in apropelling configuration in which the output shaft is drivingly engagedto the combustion engine via the first drive path and in which the firstdrive path is drivingly engaged to the second drive path.

Embodiments E and F may include any of the following elements, in anycombinations:

Element 3-1: the planetary gear train defines a first input drivinglyengageable to the electric motor, a second input drivingly engageable tothe combustion engine, a first output drivingly engageable to thecompressor, and a second output drivingly engageable to the outputshaft, the first input drivingly engaged to the first output while beingdisengaged from the second output in the APU mode, the second input indriving engagement with the second output in the propulsion mode.Element 3-2: the first and second inputs are in driving engagement withthe first and second outputs in the propulsion mode. Element 3-3: theplanetary gear train includes a brake for engaging the first input tothe output shaft in the propulsion mode. Element 3-4: the planetary geartrain includes a sun gear, planet gears meshed with the sun gear androtatably supported on a planet carrier, and a ring gear meshed with theplanet gears, the first input and the first output defined by a firstone of the sun gear, the planet carrier, and the ring gear, the secondinput and the second output defined by a second one of the sun gear, theplanet carrier, and the ring gear, rotation of a third one of the sungear, the planet carrier, and the ring gear being limited in thepropulsion mode and allowed in the APU mode. Element 3-5: the planetarygear train includes a sun gear, planet gears meshed with the sun gearand rotatably supported on a planet carrier, and a ring gear meshed withthe planet gears, the first input, the first output, and the secondinput are defined by a first one of the sun gear, the planet carrier,and the ring gear, the second output defined by a second one of the sungear, the planet carrier, and the ring gear, rotation of a third one ofthe sun gear, the planet carrier, and the ring gear being limited in thepropulsion mode and allowed in the APU mode. Element 3-6: the secondinput is in driving engageable to the combustion engine via a one-wayclutch such that, in the APU mode, the second input is disengaged fromthe combustion engine. Element 3-7: the second input is in drivingengageable to the combustion engine via a clutch having an engagedconfiguration in which the combustion engine is drivingly engaged to thesecond input and a disengaged configuration in which the combustionengine is disengaged from the second input. Element 3-8: the planetarygear train includes first and second planetary gearboxes drivinglyengaged to one another, the first planetary gear box having a first sungear, first planet gears meshed with the first sun gear and rotatablysupported on a first planet carrier, and a first ring gear meshed withthe first planet gears, the second planetary gearbox including a secondsun gear, second planet gears meshed with the second sun gear androtatably supported on a second planet carrier, and a second ring gearmeshed with the second planet gears, the electric motor and thecompressor drivingly engaged to the first planetary gearbox, thecombustion engine drivingly engaged to the second planetary gearbox, abraking system operable in a first configuration corresponding to theAPU mode and a second configuration corresponding to the propulsionmode. Element 3-9: the electric motor and the compressor are drivinglyengaged to the first sun gear, the first sun gear drivingly engaged tothe second sun gear via a common shaft, the combustion engine drivinglyengaged to the second ring gear, the output shaft drivingly engaged tothe first carrier. Element 3-10: the electric motor and the compressorare drivingly engaged to the first ring gear, the first ring geardrivingly engaged to the second ring gear, the combustion enginedrivingly engaged to the second carrier, the output shaft drivinglyengaged to the first carrier. Element 3-11: the first ring gear and thesecond ring gear are two portions of a common ring gear. Element 3-12:the first ring gear is drivingly engaged to the second ring gear viaidler gears. Element 3-13: the braking system includes a first brakeoperatively connected to the first ring gear, a second brake operativelyconnected to the first carrier, a third brake operatively connected tothe second ring gear and a fourth brake operatively connected to thesecond carrier. Element 3-14: the planetary gear train defines a firstinput drivingly engageable to the combustion engine, a second inputdrivingly engaged to the electric motor, a first output in drivingengagement with the output shaft, and a second output in drivingengagement with the compressor, the first drive path extending from thefirst input to the first output, the second drive path extending fromthe second input to the second output, the second input drivinglyengaged to the second output while being disengaged from the firstoutput in the APU mode, the first input in driving engagement with thefirst output in the propulsion mode. Element 3-15: the planetary geartrain includes a sun gear, planet gears meshed with the sun gear androtatably supported on a planet carrier, and a ring gear meshed with theplanet gears, the first input and the first output defined by a firstone of the sun gear, the planet carrier, and the ring gear, the secondinput and the second output defined by a second one of the sun gear, theplanet carrier, and the ring gear, rotation of a third one of the sungear, the planet carrier, and the ring gear being limited in thepropulsion mode and allowed in the APU mode. Element 3-16: the planetarygear train includes a sun gear, planet gears meshed with the sun gearand rotatably supported on a planet carrier, and a ring gear meshed withthe planet gears, the output shaft drivingly engaged to the carrier, thefirst drive path including the carrier, the second drive path includingthe ring gear, a brake having an engaged configuration in which rotationof the sun gear is limited in the propulsion mode and a disengagedconfiguration in which rotation of the sun gear is allowed in the APUmode. Element 3-17: the planetary gear train includes a sun gear, planetgears meshed with the sun gear and rotatably supported on a planetcarrier, and a ring gear meshed with the planet gears, the output shaftdrivingly engaged to the carrier, the first drive path including thering gear and the carrier, the second drive path including the ringgear, a brake having an engaged configuration in which rotation of thesun gear is limited in the propulsion mode and a disengagedconfiguration in which rotation of the sun gear is allowed in the APUmode. Element 3-18: a one-way clutch between the combustion engine andthe ring gear.

The embodiments described in this document provide non-limiting examplesof possible implementations of the present technology. Upon review ofthe present disclosure, a person of ordinary skill in the art willrecognize that changes may be made to the embodiments described hereinwithout departing from the scope of the present technology. Yet furthermodifications could be implemented by a person of ordinary skill in theart in view of the present disclosure, which modifications would bewithin the scope of the present technology.

The invention claimed is:
 1. An aircraft power plant comprising: a transmission having a first transmission drive path and a second transmission drive path, the second transmission drive path selectively drivingly engageable to the first transmission drive path, an electric motor in driving engagement with the first transmission drive path, a compressor in driving engagement with the first transmission drive path, a combustion engine in driving engagement with the second transmission drive path, and an output shaft drivingly connectable to a thrust generator for propelling an aircraft equipped with the aircraft power plant, the output shaft in driving engagement with the second transmission drive path.
 2. The aircraft power plant of claim 1, wherein the transmission includes an engagement device operable in an Auxiliary Power Unit (APU) mode in which the first transmission drive path is disengaged from the second transmission drive path and in which the compressor is driven by the electric motor and in a propulsion mode in which the first transmission drive path is drivingly engaged to the second transmission drive path and in which both of the combustion engine and the electric motor are drivingly engaged to the compressor and to the output shaft.
 3. The aircraft power plant of claim 2, wherein the engagement device is an electromechanical clutch having an engaged configuration corresponding to the propulsion mode and in which the combustion engine is drivingly engaged to the compressor and having a disengaged configuration corresponding to the APU mode and in which the combustion engine is disengaged from the compressor.
 4. The aircraft power plant of claim 2, wherein the engagement device is a one-way clutch via which a rotational input of the combustion engine is transmittable to the compressor and via which a rotational input from the electric motor is isolated from the output shaft.
 5. The aircraft power plant of claim 1, further comprising a turbocharger having a turbocharger compressor and a turbocharger turbine, the turbocharger compressor having an outlet fluidly connected to an air inlet of the combustion engine, the turbocharger turbine having an inlet fluidly connected to an exhaust of the combustion engine.
 6. The aircraft power plant of claim 5, wherein the turbocharger is mounted on a shaft in driving engagement with the second transmission drive path.
 7. The aircraft power plant of claim 1, further comprising a second electric motor in driving engagement with the second transmission drive path.
 8. The aircraft power plant of claim 1, wherein a brake is operatively connected to the output shaft, the brake in engagement with the output shaft when the first transmission drive path is disengaged form the second transmission drive path.
 9. The aircraft power plant of claim 1, wherein the second transmission drive path is engageable to the first transmission drive path via a first clutch and a second clutch, the first clutch selectively engaging the combustion engine to the first transmission drive path, the second clutch selectively engaging the output shaft to first transmission drive path.
 10. The aircraft power plant of claim 1, wherein the electric motor is a motor/generator, the combustion engine driving the motor/generator in a generator mode when the first transmission drive path is drivingly engaged to the second transmission drive path.
 11. An aircraft power plant comprising: a transmission defining a drive path, the transmission having a propulsion mode and an Auxiliary Power Unit (APU) mode; an electric motor; an output shaft drivingly connectable to a thrust generator for propelling an aircraft equipped with the aircraft power plant, the output shaft drivingly engaged to the drive path in the propulsion mode; a combustion engine; and a compressor drivingly engaged to the electric motor via the drive path, the output shaft and the combustion engine drivingly engaged to the drive path in the propulsion mode of the transmission and disengaged from the drive path in the Auxiliary Power Unit (APU) mode.
 12. The aircraft power plant of claim 11, wherein the transmission includes an engagement device having a disengaged configuration in which the drive path is disengaged from the combustion engine and the output shaft while the compressor is driven by the electric motor and an engaged configuration in which the drive path is drivingly engaged to the combustion engine and the output shaft.
 13. The aircraft power plant of claim 12, wherein the engagement device is one of an electromechanical clutch and a sprag clutch.
 14. The aircraft power plant of claim 11, further comprising a turbocharger having a turbocharger compressor and a turbocharger turbine, the turbocharger compressor having an outlet fluidly connected to an air inlet of the combustion engine, the turbocharger turbine having an inlet fluidly connected to an exhaust of the combustion engine.
 15. The aircraft power plant of claim 14, wherein the turbocharger is mounted on a shaft drivingly engaged to the output shaft.
 16. The aircraft power plant of claim 11, further comprising a second electric motor in driving engagement with the output shaft.
 17. The aircraft power plant of claim 1, wherein the combustion engine and the output shaft are engageable to the drive path via a first clutch and a second clutch.
 18. The aircraft power plant of claim 11, wherein the electric motor is a motor/generator, the combustion engine driving the motor/generator in a generator mode when the drive path is drivingly engaged to the combustion engine.
 19. A method of operating an aircraft power plant having a hybrid propulsion system including an electric motor, an output shaft drivingly connectable to a thrust generator, a combustion engine, and a compressor, the method comprising: in an Auxiliary Power Unit (APU) mode, transmitting a first rotational input from the electric motor to the compressor along a drive path while maintaining a second rotational input from the combustion engine separate from the output shaft; and in a propelling mode, transmitting the second rotational input to the output shaft via the drive path to drive the output shaft with both of the combustion engine and the electric motor.
 20. The method of claim 19, wherein transmitting the second rotational input to the output shaft includes drivingly engaging the combustion engine to the drive path via a clutch. 